Quasi-nuclear and quark model baryonium: historical survey

In the 70’s, there has been many indications of new mesons coupled to the nucleon–antinucleon (NN) system. States below the NN threshold were claimed, e.g., in radiative transitions NN → X + γ. Above the threshold, states were tentatively seen as bumps in cross sections. Clarifying the experimental situation was one of the main motivation for building the lowenergy part of the antiproton beam facility at CERN. Most candidates for baryonium have not been confirmed by careful scans. It remains, however, • Some of the multimeson states seen below threshold in annihilation experiments might have to do with baryonium. This is the case in particular for the state called “AX” (now more prosaically f2(1565) [1]), as pointed out by Dover [2].